Mutual coupling means the electromagnetic interaction of nearby antenna elements in a multi-antenna system. The currents in each element couple electromagnetically to the neighboring elements thus distorting the ideal current distributions along the elements. This causes changes in the radiation patterns and also in the input impedances of the antennas. From the RF point of view, isolation between the feeding ports of the antennas and mutual coupling are the same thing. So low isolation means high coupling causing energy transfer between the ports and, therefore, decrease in the efficiencies of the antennas. The strength of the isolation can be measured by looking at the scattering (S-) parameters of the antennas. So, for example, the S-parameter S21 determines how much energy is leaking from port 1 to port 2.
Furthermore, a typical mobile phone antenna is generally compounded of a resonating antenna element and a more or less resonating chassis of the phone, working as a positive pole and a negative pole of the antenna, respectively. This generalization is valid regardless of the type of the antenna element. In practice, the ground plane of the PWB (printed wiring board) also works as the main ground for the antenna and, depending on the inner structure of the phone, the currents induced by the antenna extend over the whole chassis. On the PWB the currents are concentrated on the edges.
Modern phone terminals are designed to operate in several cellular and also non-cellular systems. Therefore, the terminals must also include several antenna elements in order to cover all the desired frequency bands. In some cases even two antennas working at the same frequency band are required for optimizing the performance. In small terminals the antenna elements are located very close to each other thus leading to a low natural isolation. This problem arises especially at low frequencies, where the electrical size of the terminal is small, and when the coupled antennas work at the same frequency band. Moreover, the antennas are also connected galvanically via the PWB acting as a mutual ground plane for the antennas.
Furthermore, the performance of a mobile phone antenna depends strongly on a size of the PWB. Optimal performance is achieved when the size coincides with certain resonance dimensions, i.e., when the width and the length of the PWB are suitably chosen compared with wavelength. Therefore, an optimal size for the PWB depends on the frequency. A non-resonating ground plane causes significant reduction in the impedance bandwidth and in the efficiency of the antenna. On the other hand, the currents on a resonating ground plane are strong causing significant electromagnetic coupling between the antenna and the other RF-parts of the phone. Furthermore, the strong chassis currents also define the locations of the SAR (specific absorption rate) maximums.
Furthermore, mobile phones have been designed mainly in a mono block form but demands from customers for a variety of forms are increasing. Fold phones are extremely popular already in Asia and they are getting popular year by year in Europe and America. Slide phones have also joined the competition. From antenna design point of view, moving from the mono block form to the fold or slide form adds extra complexity and difficulties for achieving an adequate performance at all possible modes of operation of a fold/slide device.
Because small antenna on mobile phones is heavily relying on its chassis dimension to work as an important part of the antenna length, an antenna performance changes dramatically when the fold/slide phone changes its modes from open to close. That makes the antenna design very difficult and forces a designer either to optimize the design for one mode while sacrificing for another or compromise at both modes to find a good balance. Inserting series inductors at the connection of lower and upper parts of the phone is one known prior art solution to the problem. It isolates lower and upper parts from an RF point of view. But it requires a large area on the PWB to accommodate numbers of inductors for each line connecting upper and lower halves. Insulating a metallic hinge also remains problematic.